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<h1 class="topictitle1">A VM or Disk Fails to Be Restored</h1>
<div><div class="section"><h4 class="sectiontitle">Symptom</h4><p>A VM or disk fails to be restored. The error message "Failed to restore the VM disk." is displayed in the job details, and <strong>Write failed, err:'Unknown error'</strong> is recorded in the log.</p>
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<div class="section"><h4 class="sectiontitle">Possible Causes</h4><p>In SAN transmission mode, if the copy used for restoration contains data of thick provision lazy zeroed or thin provisioned disks, write I/O errors may occur. (For details, visit VMware Knowledge Base: <a href="https://kb.vmware.com/s/article/52193" target="_blank" rel="noopener noreferrer">https://kb.vmware.com/s/article/52193</a>.)</p>
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<div class="section"><h4 class="sectiontitle">Troubleshooting</h4><ol><li><span>Choose <span class="uicontrol" id="EN-US_TOPIC_0000002164813998__en-us_topic_0000002199964685_uicontrol1812933174613"><b><span id="EN-US_TOPIC_0000002164813998__en-us_topic_0000002199964685_text0129103112462"><strong>Explore</strong></span> &gt; <span id="EN-US_TOPIC_0000002164813998__en-us_topic_0000002199964685_text81291431134613"><strong>Copy Data</strong></span> &gt; <span id="EN-US_TOPIC_0000002164813998__en-us_topic_0000002199964685_text860175510430"><strong>Virtualization</strong></span> &gt; <span id="EN-US_TOPIC_0000002164813998__en-us_topic_0000002199964685_text8319371528"><strong>VMware</strong></span></b></span>.</span></li><li><span>On the <span class="wintitle"><b>Copy</b></span> tab page, select the copy used for restoration.</span><p><ul><li>VM restoration: Choose <strong>Operation</strong> &gt; <strong>Restore</strong>. On the displayed page, enable <span class="uicontrol"><b>NBDSSL-based Restore</b></span>.</li><li>Disk restoration: Select the disk to be restored and click <strong>Restore</strong>. On the <strong>Restore</strong> page, enable <span class="uicontrol"><b>NBDSSL-based Restore</b></span>.</li></ul>
</p></li><li><span>Click <span class="uicontrol"><b>OK</b></span> and start the restoration job.</span></li></ol>
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<div class="parentlink"><strong>Parent topic:</strong> <a href="en-us_topic_0000002200054965.html">FAQs</a></div>
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